Mac or Linux - doenst even hae to be a new mac, probbaly be better to run snow leopard or lion over mavericks. Either that or Linux.
Being PC comptible is obvverated in the non-techie/retired generation. Most do email facebooks, some surfing and some light word processing. If they do thier own accounting then it would be the Mac as it has quicken/quickbooks.
But for most folks Linux would do the job. Both OSs are less suceptible to malware and do a great job for the jobs most older gernation would put it to.